There are three ways in which human behaviour may modify the atmosphere, either deliberately or inadvertently: by changing the concentration of constituent substances (including water); by releasing heat; and by changing the physical and biological properties of the earth's surface. As a result of these changes, people and other living organisms may be exposed to harmful levels of toxic pollutants transported by the atmosphere. Droughts and desertification may occur. Changes in the concentration of ozone in the stratosphere may alter the amount of solar ultraviolet light reaching the surface of the earth, with possible effects on the health of humans and other species. Or the climate may be altered, with either beneficial or detrimental consequences. All such possibilities have caused concern during recent years and, as is characteristic of atmospheric environmental problems, the induced changes have been regional or global in scale and hence affected large numbers of people.
There can be a time lag of the order of decades between the emission of gases into the atmosphere and their full manifestation in atmospheric and biological consequences. Past emissions have already committed planet Earth to a significant warming, radiation load and other effects. It is apparent that, because of lack of understanding of the basic global cycles of carbon, sulphur and associated elements, long-term climatic changes cannot be predicted with confidence; even if the postulated general warming due to the increase in carbon dioxide concentrations were actually to take place, it would be impossible to forecast its full effects on climate patterns – and all the more so its economic and social consequences. The climate changes probably would not be uniform, and the social economic consequences would be likely to benefit some areas and work to the detriment of others.
During the 1970s a number of atmospheric alterations due to various human activities were observed:< Carbon dioxide concentrations slowly and steadily increased, chiefly as a result of the increasing use of fossil fuels but also due to forest clearing. This has, as yet incompletely understood, implications for world weather conditions and agriculture, because it alters temperature and precipitation patterns and the distribution of snow and ice cover. It is part of the phenomenon known as global warming.
Acid rain became an established phenomenon that results from the long-distance transport in the atmosphere of sulphur oxides and nitrogen oxides, produced primarily by fossil fuel combustion. One of its adverse effects is the acidification of inland waters.
Photochemical oxidants, which cause smog, decreased in cities with effective controls, and increased where controls were absent or ineffective or where automobile use increased.
Sulphur dioxide and suspended particulate matter concentrations decreased in most cities with control policies, but increased elsewhere, especially in developing countries.
Stratospheric particulates appear to have increased somewhat, with possible undetermined effects on climate.
While local climatic changes occurred (as in heat islands and hazy areas), the question of whether long-term climatic changes are in progress, and if so at what rate, remains controversial.